O! I guess I missed that it is REPLICATION and not SYNCHRONIZATION!
So my updates/upgrades to design documents in application_code DB can
be propagated to clients that choose to upgrade and will not impact
other "custom" design documents.
This sounds like a very good approach. Thanks.
Just to explore it a bit more: is it possible to solve this with
JavaScript includes? (make validation function to point to a standard
JavaScript function available in some centralized location)